Many well completions involve setting a liner or casing string in a portion of the well bore. In some extended reach wells, such as wells drilled from platforms or "islands," a string must be set in a slant drilled (i.e., inclined angle) portion of a deviated hole. The inclined portion is located below an initial (top) portion of a lesser inclined angle. The angle (from vertical) of these inclined holes frequently approaches 90 degrees (i.e., the horizontal) and sometimes exceeds 90 degrees. The result is a well bottom laterally offset from the top by a significant distance. Current state-of-the-art allows extensive drilling of well bores at almost any angle, but current well completion methods have experienced problems, especially related to the setting of casing or liner strings in long, highly deviated well bores.
The liner or casing string is set in a pre-drilled hole. The drill string and bit used to cut the hole is rotated, thereby reducing drag forces which retard the pipe string from sliding into the hole. The diameter and weight of the casing/liner string being set is larger and heavier than the drill string. Because of this, the torsional forces needed to rotate the casing or liner can be greater than the torsional strength of the pipe itself, or greater than the available rotary torque. Casing or liner strings are therefore normally run (i.e., slid) into the hole without drag reducing rotation.
Running in deviated holes can result in significantly increased (high) drag forces. A deviated hole portion is defined as one having an axis in a direction at a significant incline angle to the vertical or gravity direction. A casing or liner pipe string may become differentially stuck before reaching the desired setting depth during running into a deviated or high drag hole, especially if the incline angle exceeds a critical angle where the weight of the casing or liner in the wellbore produces more drag force than the component of weight tending to slide the casing or liner down the hole. If sufficient additional force (up or down) cannot be applied, the result will be stuck pipe string and possible effective loss of the well. Even if a stuck string is avoided, the forces needed to overcome high drag may cause serious damage to the pipe. These problems are especially severe for wells with long, nearly horizontal (i.e., an incline angle of nearly 90 degrees) intervals.
Long, nearly horizontal well intervals may be needed for fluid production from tight and/or thin bed reservoirs or from fields having limited surface access. For example, an offshore drilling site may be unlicensable or excessively costly. The ability to drill from an on-shore site to an offshore resource horizontally displaced from the drilling site by several kilometers may mean the difference between an unavailable and a producing resource.
Even for fields where reservoir access (or permeability) is not a problem, long nearly horizontal well portions may be economically desirable because of higher production rates. Higher production rates may be possible in horizontal well portions from zones where production of unwanted fluids (such as water/gas in oil fields) from adjacent beds, normally occurs in vertical wells, i.e., coning.
Common casing or liner running (i.e., installation) methods to overcome increased drag in a deviated well portion either 1) add downward force or 2) reduce the coefficient of friction, e.g., by lubrication. A modification of the added force approach provides bumpers to deliver downward shocks and blows in addition to added downward static forces.
However, only a limited downward force can be exerted on the pipe string. Excessive downward force can convert a pipe string (normally supported from the top of the well) into a highly compressed member. Compression tends to buckle the string, adding still further drag forces (if laterally supported by the well bore) or causing structural failure (if laterally unsupported). In addition, large amounts of added downward force may be impractical.
Similar limits affect common lubricating or coefficient of friction reducing methods since the coefficient of friction cannot be reduced to zero. These lubricating methods do allow longer pipe strings to be run into a deviated hole. However, as longer lubricated pipe strings are run into the deviated well, unacceptable drag forces will still be generated. The geometry and drilled surface conditions of some holes may also create increased resistance (high drag) conditions in shorter inclined holes, even if lubricating methods are used.
A flotation method of placing a pipe string into a deviated, liquid filled hole is also known. This method is illustrated in U.S. Pat. No. 4,384,616. After providing a means to plug the ends of a pipe string portion, the plugable portion is filled with a low density, miscible fluid to provide a buoyant force. The low density fluid must be miscible with the well bore fluids and the formation. Miscibility is required to avoid a burp or "kick" to or from the formation outside the pipe string when plugged portion fluid is discharged to the formation/well bore. Circulation of drilling mud is also not possible during running or feeding the plugged string into the wellbore. After feeding the plugged string into the well bore, the plugs are drilled out and the low density miscible fluid is forced into the well bore/pipe annulus. Further casing operations, if any, (i.e., cementing) are accomplished without the assistance of a low density miscible fluid providing a buoyant force.
The known string flotation method requires added risk and well completion steps, especially if cementing is required. The low density fluids compatible with the formation and bore fluid must be circulated out ahead of a cement slurry. This requires drilling out the plug(s) prior to cementing of the casing or liner string. Subsequent to the cementing, a second drilling out (of hardened residual cement) is frequently also required. The multiple drilling steps result in costly well completions and increase the risk of damage to the pipe string and formation.
None of the current approaches known to the inventors allow the flotation of a string into a high drag slanted well without a multi-step completion process. The cost of the miscible fluid and multi-step completion process has apparently resulted in little or no commercially practical application of the current flotation method.
A simplified flotation device and method are needed to allow the placement and completion of long pipe strings in extended reach well bores. The method and device should also be safe, reliable, and cost effective.